1. Field of the Invention
This invention relates to methods of and apparatus for measurement of picosecond semiconductor laser pulse duration using the internally generated second harmonic emission accompanying the laser output. Accordingly, it is a general object of this invention to provide new and improved methods and apparatus of such character.
2. General Background
We note that, in order to achieve high accuracy, background subtraction for the fundamental power and appropriate choice of reference signal may be necessary. Ideally, a pulse goes from about 0, to a maximum, down to about 0 again. In actuality, however, the pulse does not go from 0 to a maximum to 0, but instead goes to some background level--a dc level. That dc level should be subtracted in calculations. That subtracted amount is termed "background subtraction". In the present invention, P.sub..omega. for the picosecond pulses is the total average power without background subtraction, because the ratio between the energy contained in the pulses and the background is very high (greater than 20 to 1). This high contrast ratio is attributed to the index-guided laser structure and low threshold current. This ratio, however, is much lower for pulses generated from the gain-guided lasers with high threshold current, as described by P. L. Liu, C. Lin, I. P. Kaminow and J. J. Hsieh, IEEE. J. Quantam, Electron, QE-17, 671 (1981). In those cases, the background energy needs to be subtracted from P.sub..omega..
The efficiency of second harmonic generation depends on the number of longitudinal modes in the lasing spectra. If the fundamental power I is equally distributed over N longitudinal modes, the second harmonic power is proportional to (2-N.sup.-1)I.sup.2, as described by N. Bloembergen, Nonlinear Optics, (W. A. Benjamin, Inc., Reading, Mass., 1965).
There have been several reports on the properties of the second harmonic emission in gallium arsenide and in InGaAsP lasers as shown by L. D. Malmstrom, J. J. Schlikman and R. H. Kingston, J. Appl. Phys., 35 248 (1964) and by T. Furuse and I. Sakuma, Opt. Commun. 35, 413 (1980) In conventional semiconductor lasers, the epitaxial layers are grown in the (100) plane and the cleaved facets are in the (110) plane. The symmetry of the laser materials requires that the TE laser emission, the normal operating mode, generates second harmonic emission, polarized in the direction normal to the junction plane, while the TM laser emission generates no second harmonic emission.
Ultrashort optical pulse generation with semiconductors lasers is of interest for such applications as high bit rate optical communications and very fast data processing. In the picosecond regime, the most commonly used technique of pulse duration measurement has been the nonlinear intensity autocorrelation technique involving phase-matched second harmonic generation in a nonlinear optical crystal, such as a LiIO.sub.3 crystal.
In general, intensity autocorrelation is well-known. The intensity autocorrelation technique involves the provision of a very sharp pulse, such as a picosecond pulse, splitting it in half, delaying one half of the split pulse while not delaying the other, then combining the two pulses. The pulses are combined and passed through a nonlinear crystal to generate second harmonics. The intensity of the second harmonics is higher when the two pulses overlap. By delaying one pulse, relative to the other, an overlap can occur. The delay is physically measured. If the delay is, for example, one centimeter, the time duration can be calculated since it is known that the speed of light can travel one centimeter in 30 picoseconds, approximately.